1. Field of the Invention
The present invention relates to an antenna, and more particularly, to an antenna for use in a portable telephone such as a cellular phone and a manufacturing method thereof.
2. Description of the Related Art
Portable phones are getting smaller in size due to the developments in battery technology and the integration of internal circuits thereof into chips. In addition, antennas employed in the portable phones are getting smaller along with the main body of the phones. Although the antenna of the portable phone is getting smaller, it is necessary that it still exhibit enough sensitivity to operate properly. In order to minimize its size while maintaining the sensitivity thereof, the antenna assembly for the portable phone typically employs a helical antenna incorporated with a whip antenna.
FIGS. 1 and 2 illustrate an example of a conventional antenna assembly which employs a helical antenna and a whip antenna, in an extended position and a retracted position, respectively. In the antenna assembly of FIGS. 1 and 2, the helical antenna includes a helical element 10, a first metal fitting 12, a second metal fitting 14, and a first antenna cover 16. The helical element 10 is usually made of an elastic metal and has an electrical length of .lambda./4. The first metal fitting 12 has an aperture passing through its center vertically and is threaded on its outer surface. The second metal fitting 14 provides an electrical connection between the helical element 10 and the first metal fitting 12. The first antenna cover 16 encloses the helical element 10 to prevent the performance of the helical element 10 from being deteriorated due to deformation, damage, or oxidation thereof caused by external contact or impact. Meanwhile, the whip antenna includes an antenna rod 20, a second cover 22, and a conductive stopper 24. The antenna rod 20 has an electrical length of .lambda.4 and operates as a monopole antenna. The second cover 22, which is made of a nonconductive plastic material, encloses the antenna rod 20 to protect it from external contact. The stopper 24 is attached to the bottom end of the antenna rod 20. In FIGS. 1 and 2, reference numeral 36 denotes a housing body of a portable phone.
In such an antenna assembly, the whip antenna is installed capable of upward and downward movement. When a user extends the whip antenna by pulling a knob 26 installed at the upper end thereof as show in FIG. 1, the power from a signal processing circuit 30 inside the phone is provided to the antenna rod 20 via an antenna clip 32, a housing fitting 34, and the stopper 24, and simultaneously to the helical element 10 via the first metal fitting 12 and the second metal fitting 14. In this operation mode, the whip antenna mainly operates as a monopole antenna, and the helical antenna operates as an accessory of the whip antenna. Meanwhile, when the antenna is retracted into the housing body as shown in FIG. 2, power from the signal processing circuit 30 is provided only to the helical element 10 since the stopper 24 is separated from the housing fitting 34 and the second metal fitting 12. Therefore, only the helical antenna receives or transmits signals.
Typically, the helical element of the antenna assembly described above is manufactured by winding an elastic metal wire over a dielectric such as polyvinyl chloride (PVC). However, this method of manufacture is quite costly because the elastic metal for the helical element is expensive as compared with a common material such as copper. Meanwhile, a metal wire used for the helical element is required to be highly oxidation-resistant so that the helical element is resistant from oxidization inside the antenna cover. To solve the oxidation problem, the outer surface of the metal wire may be coated with a plastic or a polymer. However, an accurate coating of the outer surface of the metal wire requires several additional processing steps. These additional processing steps significantly decrease the efficiency of mass-production of the antenna. Furthermore, the conventional helical antenna may be deformed or lose a required antenna characteristics when it experiences a strong impact even though it is protected by the antenna cover.